The Content-Encoding of the page is unrelated to the Content-Type.
Content-Encoding specifies how the content is compressed (i.e. gzip, deflate, br...) and thus how it should be decompressed. Content-Type instead specifies how the content should be interpreted, i.e. as html, image, PDF etc. And for text data like plain text or HTML it can additionally specify the character set and encoding, i.e. UTF-8, ISO-8859-1 or similar.
If no Content-Type is specified browsers try to guess what the type and character set/encoding the content might be. This can have unintended side effects because different browsers might guess differently and specifically the dedected character set might also depend on various contexts like the character set of an embedding frame.
Thus you should always specify a Content-Type and also set the Charset parameter for text types. Additionally you should set the HTTP header
X-Content-Type-Options: nosniff so that the browser does not try to guess the type even though the declared is differently from the guessed type.
For more information about the problem with MIME sniffing see also MIME Sniffing: feature or vulnerability, Attacks on browser-based content sniffing, Secure Content Sniffing for Web Browsers or
How to Stop Papers from Reviewing Themselves.